Climate Science Glossary

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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

How sensitive is our climate?

What the science says...

Net positive feedback is confirmed by many different lines of evidence.

Climate Myth...

Climate sensitivity is low
"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)

Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we double the amount of carbon dioxide in the atmosphere. This includes feedbacks which can either amplify or dampen that warming. This is very important because if it is low, as some climate 'skeptics' argue, then the planet will warm slowly and we will have more time to react and adapt. If sensitivity is high, then we could be in for a very bad time indeed.

There are two ways of working out what climate sensitivity is. The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence, by looking at climate changes in the distant past:

Various paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras. Adapted from PALEOSENS (2012) Figure 3a by John Cook.

These calculations use data from sources like ice cores to work out how much additional heat the doubling of greenhouse gases will produce. These estimates are very consistent, finding between 2 and 4.5°C global surface warming in response to doubled carbon dioxide.

It’s all a matter of degree

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against reducing greenhouse gas emissions because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

Comments

RW1 - To use a database like HITRAN, you set up your parameters (in whatever spectral software you are using, such as JavaHAWKS) for two different conditions, run it twice, and look at the differences between the outputs.

The output of interest is the summed energy radiated from the atmosphere given a particular surface temperature and atmospheric mix. The difference between them (~3.6 W/m^2 for doubling CO2 with HITRAN data, 3.7 for more up to date models) is the difference in total radiated energy - outgoing energy. Not isotropic radiation from a particular level of the atmosphere, but the difference in total emissions. Changes in atmosphere modify the emissivity of the Earth, as per the Stefan–Boltzmann law; the amount of thermal radiation emitted at any particular temperature. And that leads to imbalances with incoming sunlight that result in climate changes as energy accumulates or leaves.

It's as simple as that - what is the sum difference between radiated powers after an atmospheric change. That 3.6/3.7 Watts is the integrated difference in total radiation going out to space at a particular temperature - which is the very definition of "radiative imbalance".

RW1 - you are asking for documentation of what is implicit in the equations. Lets see if I can attempt it.

At heart of equations, you consider a small slice of atmosphere. It has radiation from in from below, (from surface and lower layers) and from above (from upper layers in atmosphere). The equations capture absorption, transmission, emission (in ALL directions - which of course is the inputs to layers above and below) for a given gas composition, P,T. The integral of all the layers is what then allows you to calculate what comes out of the top. All the interaction is captured. You know it correct because the model results agree with empirical measurement.

Sigh. The Science of Doom takes you through text book. Is that documentation enough? The problem seems to be that you are looking for a statement that doesnt exist because it would make no sense. The way real physics is done is bears little relationship to way you are trying to approach it. We are trying desperately to show why that is. As far as I can see you either:
a/ study the physics
b/ see that since model matches measurement so model must be right.

I am guess that are ignoring the textbook, SoD, papers, because they dont relate to George White's "logic" and you search in vain for an analogous treatment. However, this is the right way to do it. I'm beyond my power to help you further.

Rahmstorf 2008, linked in the Advanced version of this post, gives 3.7 W/m^2 as an undisputed figure for CO2 forcing.

Without any feedbacks, a doubling of CO2 (which amounts to a forcing of 3.7 W/m2) would result in 1°C global warming, which is easy to calculate and is undisputed. ... consensus holds that a doubling of CO2 causes a radiative forcing of 3.7 W/m2, which in equilibrium would cause 3°C±1.5°C of global warming.

muon - the issue is does "reduction in the atmospheric window" mean the same thing to RW1 as I think it means.
You can say yes, but I suspect that RW1 then has corollary from that shows a very different understanding.

scaddenp - Yeah, I thought that was a peculiar phrase, which seems to add an unnecessary layer of complication. All I did was point to the link, as it seemed (in #105) that he couldn't find it. How he chooses to interpret this particular 3.7 W/m^2 is up to him, although both KR and you have made it very clear.

I believe the "atmospheric window" issue is tied (again) to George White - he believes the window of IR going straight to space is >90 W/m^2, whereas Trenberth estimates 40 W/m^2, and asserts that all greenhouse gas effects operate by narrowing that window. He seems to neglect lapse rate and GHG concentration effects raising the altitude (and dropping the temperature) of emission, and in addition argues that the 90 W/m^2 represents a limiting band on GHG effects.

RW1 - The models operate by calculating upwards and downwards emissions from all levels of the atmosphere, and the 3.7 W/m^2 represents all the effects: band broadening due to higher GHG concentrations, band deepening due to higher effective altitudes of emission to space, higher reemission to the ground, etc. So the answer to your question is partially, although not readily picked out of the other effects.

RW1 - "You are saying the 3.7 W/m^2 increase is not the reduction in the atmospheric window? " Hmm, but we have this rather odd expression about "reduction in the atmosphere window". What does this mean? KR identifies it with a GW idea. Can you phrase the question in a way that we can understand, and preferably makes physical sense?

RW1 - The answers to your four questions are "Yes, ~40 W/m^2", "Yes, although a fair amount of energy also goes into the atmosphere via convection and latent heat (~20%)", "Only partially", and "Almost, it's the amount prevented from leaving via various effects - more absorption and higher/colder emissions".

Sorry, but these are obviously important questions for you, and I would be doing a disservice by giving un-nuanced answers.

The actual number for the atmospheric window is irrelevant to the particular question at hand here. Whether it's 40 W/m^2 or 90 W/m^2 - it doesn't matter, nor do I care.

The estimated 3.7 W/m^2 from 2xCO2 either represents a reduction in the atmospheric window or not. The fact you seem to be side stepping this fundamental question is quite revealing. It's a ridiculously simple and straightforward question with a simple yes or no answer.

I can see no one here is interested in getting to bottom of this, so it appears like I'll have to do some more searching around and figure out for myself.

RW1 - "The estimated 3.7 W/m^2 from 2xCO2 either represents a reduction in the atmospheric window or not."

Wrong. It's partially a reduction in the "window", and partially a reduction, a drop in the intensity, in the GHG bands - the ones already inhibited by the presence of greenhouse gases. Not yes or now, but "in part".

As GHG concentration rises, the effective emission altitude goes higher and higher in the troposphere, and hence (due to the lapse rate) comes out of colder and colder GHG's. They emit less than warmer lower GHG's - the additional altitude means that the repeated reduction in IR transmission as part gets emitted up (to higher levels) and parts down attenuate the IR levels. That and widening bands, the reduction of the window, combine to provide the 3.7 W/m^2 effect from doubling CO2.

That is why I gave a nuanced answer, one that actually answered your question without conveying incorrect information. It's not A or B - it's both.

RW1 - The answer is not "no", it is "in part". I've (repeatedly) clearly answered your question - narrowing of the atmospheric window is part of the 3.7 W/m^2, and deepening of the intercepted bands due to higher effective emission altitude is also part of the 3.7 W/m^2. It's not an either/or question!

1) When physicists refer to "the atmospheric window" they refer to a portion of the spectrum in which radiation is not absorbed, so radiation can pass through that "window" without appreciable loss or distortion. The atmosphere has several windows - one at the frequencies of visible light, another in the IR spectrum, and still others in the radio spectrum.

2) One of the atmospheric windows in the IR spectrum is in that range of frequencies where the majority of the surfaces IR radiation is emited. As a result, about 40 w/m^2 of IR radiation escapes to space without being absorbed by any atmospheric components (except clouds, if present).

3) Increasing CO2, O3 or H2O content into the atmosphere, or introducing novel GHG can narrow this window slightly, but the effect is very small.

4) Outside of the atmospheric window, IR radiation from the Earth's surface is entirely absorbed by GHGs; but

5) Those GHGs then emit radiation at the same frequency at an intensity that depends on their temperature. The IR radiation emitted towards space by GHGs is then absorbed by higher GHGs, which in turn emit radiation at an intensity depending on their temperature, which is in turn absorbed and so on until the atmosphere is thin enough for the upward emitted radiation to escape to space.

6) Because the radiation outside the atmospheric window that escapes to space is emitted high in the atmosphere, it is emitted by gases that are much cooler than the surface. Therefore, that radiation has a much lower intensity, ie, transmits much less energy than the radiation emitted from the surface at the same frequency. The difference between the energy that is radiated to space outside of the atmospheric window and the energy originally radiated from the surface at those same frequencies is the fundamental basis of the green house effect.

7) If you increase the concentration of a GHG, then the altitude at which radiation from that GHG will effectively escape to space will increase. Because the altitude has increased, the temperature of the radiating gas is lower, so the total energy radiated is also lower.

8) If you double the CO2 concentration, the atmospheric window will narrow slightly as the absorption band of CO2 widens. This does not mean no IR radiation will escape in the frequencies where the absorption band widens - it just means that the IR radiation in those frequencies will come from a higher, ergo cooler, ergo less energetic altitude, reducing the total IR energy escaping in that frequency band by about a third.

9) At the same time, IR in the frequencies of the previously existing absorption band will come from slightly higher in the atmosphere, and therefore carry less energy (because the emitting CO2 is colder).

10) The combination of these two effects will reduce the total energy leaving the atmosphere by 3.7 w/m^2

That is the full and complete answer to your questions (given space limits). It has been given to you ad nauseum above but you refuse to hear the answer because it is not framed according to the frankly fallacious model of the Green House effect used by George White. However, we cannot ignore the physics and give you answers that only make sense if framed in terms of George White's fallacious physics. If you try frame your question in terms of the actual physics, however, you will find you have already been answered repeatedly.

" And why do you use the word "power"? What is power?". It's the surest sign that you are dealing with someone who has got their education from George White. This incorrect usage has been pointed out to RW1 before.

RickG @123, scaddnp @125, being fair to RW1, power is just energy over time, and the Watt is a unit of power, not of energy. For convenience, when measuring the energy balance of the Earth, climatologists use the unit of watts/meter^2 rather than joules/second meter^2, which would be more formally correct when talking about energy.

Talking about the "power that escapes the atmosphere" rather than the "energy that escapes the atmosphere" would be peculiar; but talking about the "power that is transmitted" or the "power that passes through" the atmosphere is not, so I don't see your point.

Well, I am used to the more usual definition of power as rate of energy conversion. The GW usage just sounds so strange when used instead of energy flux. Mix it in with amplifier analogues and its a real recipe for confusion. There is a lot to said for accuracy ( though I know I am pot calling kettle black at times). On other hand, met anyone not acquainted with GW using power in this peculiar way when discussing radiative physics?

RW1 - The reduction in the atmospheric window represents only a small portion of the 3.7 W/m^2, as Tom Curtis said. Sorry I don't have exact numbers, but (as I have a day job) I haven't put in a request for the HITRAN data.

If you look at the actual spectra of top of atmosphere (TOA) emissions, you will see the GHG blocked bands:

The baseline of around 225K (around 650 microns) in the first graph represents the lapse-rate cooled greenhouse gas emission at the altitude where the IR can actually reach space without being intercepted by more GHG's. The higher this goes, the cooler the gases, the lower the temperature for emission, the lower the bottom of that curve. And hence the lower the integrated power over the entire spectra.

My question to you is: Why does it matter? What's the issue with the 'window' versus lowest temperature of fully intercepted bands? I'm genuinely curious, especially since you've been poking at that for some days now - why is the percentage involved in 'window' narrowing important relative to the total integrated power blocked by a doubling of CO2? Do you have an argument based upon 'window' size?

The reason I ask is because I don't see why the distribution would be an issue - the total energy imbalance (change in emitted energy with doubled CO2) is what is important as a forcing, rather than exact spectral distribution (and I say that as someone who works with spectrometers all the time!).

RW1 @129, if you already understand this then why are you asking a question which is almost nonsensical, and is certainly irrelevant, given that knowledge?

Your ask it again @130. However, it is irrelevant for all except the most abstruse studies. What concerns us it the total change in Outgoing IR Radiation, not the change at particular wave numbers.

It is also very difficult to calculate independently. For each wave number effected, you would need to calculate the energy flows by radiation and convection/latent heat from the surface to the top of the atmosphere, including both upwards and downwards energy transfers. Line By Line models do in fact calculate exactly that for every wave number (or small band of wave numbers depending on their resolution), so if you were to ask a scientist who regularly dealt with LBL models, they would no doubt be able to find the information you seek. But unless you can show a very good reason why it matters, I see no reason to pander to your request, anymore than I would pander to a geocentrist's request to show the gravitational impact of Mount Everest on the moon's orbit.

Given the very accurate prediction of LBL models as shown here, and the detailed discussion of that accuracy by Science of Dooom (linked by scaddenp @102 above) and the many quoted direct claims that the change in OLWR from a doubling in CO2 is 3.7 w/m^2, you have no reasonable basis to doubt that figure.

You need to come good with a very good reason as to why you doubt the 3.7 w/m^2 figure, and as to why you persist in your obtuse question.

RW1 - "I'm not getting any 'proof' here of anything, so I'm left to figure it out on my own."

Actually, I will have to disagree with you. You've been pointed to the documentation, you have statements from several people who are quite familiar with line-by-line atmospheric calculations, and even George White sees a ~3.6 W/m^2 imbalance with his own runs of the HITRAN code.

3.7 or so W/m^2 is the difference in total planetary emissions upon doubling CO2, the amount of extra IR not leaving at a particular temperature, the change in outward directed energy.

Please - we've offered this information honestly and clearly, as the best established data available. I would suggest you consider your own reasons for not believing it, and why you are so insistent that we are wrong.

Guys, noticed RW asked at Realclimate:
"’m wondering if someone can shed some light on this subject for me. I’ve searched around at length all over and cannot find a clear answer. The 3.7 W/m^2 estimated from simulations for the increase in ‘radiative forcing’ from a doubling of atmospheric CO2 – does the 3.7 W/m^2 represent a reduction in the atmospheric window or does it represent the half directed down due to isotropic re-radiation/redistribution (meaning a reduction in the atmospheric window of 7.4 W/m^2)???"

Clearly absolutely nothing we have said has been understood at all. I doubt he will like Gavin's accurate response either.

Interesting thread. I have a question about some frequently referenced data:

I’m wondering if someone can shed some light on this subject for me. I’ve searched around at length all over and cannot find a clear answer. The 3.7 W/m^2 estimated from simulations for the increase in ‘radiative forcing’ from a doubling of atmospheric CO2 – does the 3.7 W/m^2 represent a reduction in the atmospheric window or does it represent the half directed down due to isotropic re-radiation/redistribution (meaning a reduction in the atmospheric window of 7.4 W/m^2)???"

[Response: It is the global mean change in outgoing LW flux at the tropopause (integrated over the whole spectrum) for a doubling of CO2. - gavin]

RW1 - The 3.7 W/m^2 energy imbalance from doubling CO2 is kept in the Earth climate system, atmosphere and surface.

This is the sum result of multiple absorption/emission events distributed throughout the atmosphere, as we've told you, and as (it appears) Gavin Schmidt has repeated. Each of those individually is isotropic, with nearly equal (due to horizon effects) probability of upwards or downwards.

The sum radiation change upon doubling CO2 is that a global mean of 3.7 W/m^2 less energy leaves the top of the atmosphere.

RW1 - You've repeatedly, and by multiple people, been told what the data is, and continue to argue for your (mis)perception of it. We've told you what the results are - denying the data is the unscientific approach here.

Something to think about, RW1 - which is more likely? That everyone's interpretation of LBL analysis of CO2 forcing is somehow blatantly wrong? Or that George White (not published AFAIK, certainly not in climatology) is misinterpreting the results of the model he's run? I'm not asking for an answer from you, but just for you to consider the question.

Science works by verification and observation. In this instance, to verify the Line By Line models, or the Energy Balance models (which give essentially the same results) you would need to verify the physical laws involved, ie, the Beer-Lambert Law, Kirchoff's Law, Planck's Law, Wien's Law and Stefan Boltzmann's Law, not to mention the laws of convective heat transfer in the atmosphere and heat transfer from changes of state of H2O.

Having done that, you then need to go through the models line by line to make sure they actually implement the relevant laws appropriately. You also need to have detailed records of the composition and temperature profile of the atmosphere, and confirm that they are correctly entered into the model. You also need to check the emissivity of the various compounds in the atmosphere and make sure they are correctly fed into the model.

You have been referred to sources in this discussion where you can do each one of these things, either in little detail (Science of Doom), moderate detail (relevant textbooks) or great detail (relevant scientific papers). You have ignored all of that because, apparently, we cannot find a source that encapsulates all that knowledge into just one sentence.

Having done all that, or accepted expert opinion that it was done correctly (which is the sensible approach in that none of the above is in dispute by any practicing scientist including well known skeptics such as Pielke and Spencer), you can then compare the results of the models with observation, as has been done here. In fact, line by line, if given approximately current information on atmospheric composition at each level, models have been shown to be accurate withing less than 1% - again something you have been shown in this thread. With only approximate information, the models are accurate to withing 5% or 0.2 w/m^2 for a doubling of CO2. Again this is not in dispute by any practicing scientist once a few transparent crack pots are excluded.

Your problem is not that we are not confirming to how logic or science should work. Your problem is that we are, and for some strange reason, you don't like the answer.

"co2isnotevil: February 10th, 2011 at 8:53 am
"... looking up during the day you will see both primary and secondary IR directly originating from the Sun. This is not ‘back radiation’, but forward radiation from the Sun. Trenberth likes to call this ‘back radiation’ in order to give the false impression that GHG’s radiate this much."
(from the joannenova thread still in progress).
____________
'The question is,' said Alice, 'whether you can make words mean so many different things.'
'The question is,' said Humpty Dumpty, 'which is to be master ...'

Oh, heck, RW/GW were answered previously e.g. http://skepticalscience.com/news.php?p=5&t=216&&n=588#41029 and earlier. Sorry, I didn't realized I'd walked into the late stages of a thread-to-thread-to-thread Gish Gallop.

"George White's arguments are rife with errors. (There was going to be a third and fourth post on his errors, but the page containing his essential argument is currently down.) One of the most egregious is the halving of the reduction in outgoing radiation due to IR gases. This is very easily verified for your self using the modtran model hosted by David Archer. This is an obsolete model available on the public domain, but it still shows a change in TOA OLR of -3.17 w/m^2 for a doubling of CO2 from the default settings. Note, that is the reduction in the Outgoing Longwave Radiation, it is not "the amount of IR radiation captured" or some other vague term designed to confuse. Based on this model, with 375 ppm CO2, approx 287.8 w/m^2, while with 750 ppm, approx 284.7 w/m^2 leaves the planet."

How can 287 W/m^2 be leaving the planet? From Stefan-Boltzman, 287 W/m^2 = 266K (255K expected)?